Determination of embedded layer properties using adaptive time-frequency domain analysis

A general model for determination of the complete set of acoustical and geometrical properties of an isotropic layer embedded between isotropic or anisotropic multilayered solids is developed. These properties include density, longitudinal and shear elastic moduli, layer thickness, and loss factors, simultaneously determined from two measurements, one at normal and one at oblique incidence. The inversion model is an extension of the method proposed by Lavrentyev and Rokhlin [J. Acoust. Soc. Am. 102, 3467 (1997)] which is applicable to thick substrates. In this new method, the inversion model mimics an experiment by using the same time-domain gating of the signal reflected from the embedded layer. This allows application of this method to layered solids when reflections from different layers overlap in the time domain. The sensitivity of the method, its stability against data scatter, and the effect of the oblique incident angle are evaluated. The effect of plane-wave approximation versus beam approximation in the inverse algorithm is discussed. Experimental results are given to demonstrate examples of adhesive layer property reconstruction.